1. Field of the Invention
The present invention pertains to substrate-positioning methods and apparatuses. In particular, the invention relates to apparatuses and methods for positioning substrates in lithography systems that use optical detectors employing a signal-generating material that is substantially identical in composition to that of the substrate.
2. Background Art
Semiconductor device manufacturing processes typically require multiple uses of a lithographic apparatus to expose a substrate, e.g., a semiconductor wafer coated with photosensitive material, to a pattern contained on a reticle or mask. This exposure requires proper alignment of the previously exposed patterns on the substrate, to the new mask pattern projected on to the substrate. Proper positioning is achieved by moving the substrate holder or chuck with a stage. After alignment, the reticle may be flooded with radiation causing the mask pattern to be projected onto the photosensitive coating on the wafer surface. After development the photoresist pattern is transferred to an underlying layer of material by etching, thereby forming a device layer. Successive device layers may be formed through variations of similar exposure techniques.
Typically, semiconductor device processes require wafer and/or layer alignment. Such alignment may be carried out using a feature on the wafer, for example, an alignment mark, or a circuit feature that is easily distinguished from the adjacent circuit features. Exemplary alignment marks include chevrons, squares, crosses and grouped lines with various orientations.
Certain lithographic applications may be used to produce a three-dimensional structure that extends from a first side through to a second side of the substrate. In such cases, the substrate may be processed on one side, and then flipped over and processed on the opposite side to create the desired three-dimensional structure. Accordingly, to ensure that the connections between the features on the opposing sides are properly made, it is necessary to have some means for aligning a mask pattern to be printed on one side to an alignment feature on the opposing side.
One exemplary lithographic application requiring dual-side alignment involves the micromachining of substrates in the production of pressure transducers. Another exemplary application involves the production of optical sensors. In both cases, it may be desirable to form a conductive via running through the substrate from one side to the other to provide electrical communication therebetween. Accordingly, such lithographic applications require a dual-side alignment capability so that good electrical connections between the front and back-side electrical components may be established. A number of different approaches have been attempted to effect dual-side wafer alignment. In general, these approaches require viewing the backside of a wafer directly rather than viewing it through the substrate thickness. As used herein (unless the context clearly indicates to the contrary), the term “backside” refers to the side of the wafer that is typically in contact with the chuck and opposite the so-called “frontside,” which may be coated with a resist layer that receives the mask pattern.
As another example, U.S. Pat. No. 5,985,764 describes a technique for viewing alignment marks from the backside of a substrate by coating the alignment marks on the frontside of the substrate with an IR-reflective coating. This technique is used to overcome the topography of overlying layers, which would otherwise obscure the appearance of the alignment marks. Unfortunately, the technique requires the use of a specialized IR transparent stage and an otherwise unnecessary IR-reflective coating. As a result, the technique is a relatively complex and expensive approach for alignment marks viewing.
As a further example, U.S. Pat. No. 5,929,997 describes a method for aligning a reticle pattern with a patterned semiconductor wafer that includes simultaneous viewing of the alignment keys on a reticle and alignment targets on the backside of a wafer through the chuck supporting the wafer. However, the method is fairly complex in that the alignment light must pass through the reticle as well as through the wafer.
As a still further example, U.S. Pat. No. 6,525,805 describes an apparatus that includes a movable chuck that can position one end of an optical system to view either the frontside or backside of a substrate. This may be done for example, by placing the end adjacent the frontside of the substrate or near the frontside but outside the perimeter of the substrate. Such an arrangement allows for the optical system to be placed in optical communication with a second optical system to allow for the imaging of alignment marks on the backside onto a detector.
Accordingly, there is a need for a simpler alignment system that can image one or more alignment marks and/or features on either side of a substrate to provide for quick, precise, and inexpensive substrate positioning, alignment and processing.